Simultaneous binding of trigger factor and signal recognition particle to the E. coli ribosome

Signal recognition particle (SRP) and trigger factor (TF) both bind to ribosomal protein L23 at the peptide exit area on the 50S subunit of the E. coli ribosome. In this study, we have developed a spin-down assay and used it to estimate KD values and the corresponding enthalpies for the binding of radio-labelled SRP and TF to naked ribosomes and to ribosomes carrying a tetrapeptidyl-tRNA in the P site. At 20 degrees C, the KD value for TF binding is 2 microM and for SRP it is 170 nM for naked as well as for translating ribosomes. At 4 degrees C, the KD values for TF and SRP binding are 1.1 microM and 90 nM, respectively. Competition binding experiments reveal that SRP and TF bind simultaneously to the ribosome with little affinity interference, showing that the factors have separate binding sites on L23. This makes an alternating binding mode for TF and SRP less plausible.